Polymer electrolyte fuel cell and method for operation thereof

ABSTRACT

A decrease in voltage in a polymer electrolyte fuel cell comprising stack of unit cells caused by the temperature difference between the cells located at the ends and the other cells due to a differential in heat dissipation from end plates is prevented by controlling the cooling temperature of the cells closest to the end plates of the fuel cell without affecting the output voltage of the cells in the middle by not including a coolant flow channel in the conductive separator plate between at least one of the end plates and the unit cell located closest to the one of the end plates.

FIELD OF INVENTION

[0001] The present invention relates to a polymer electrolyte fuel cellused for portable power sources, electric vehicle power sources,domestic cogeneration systems or the like and to a method of operationthereof

BACKGROUND OF THE INVENTION

[0002] Fuel cells generate electric power by electrochemically reactinga fuel gas containing hydrogen with an oxidant gas containing oxygensuch as air through a polymer electrolyte membrane that selectivelytransports hydrogen ions. Fuel cells generally have a laminatedstructure in which a large number of unit cells are stacked. Whenoperated, fuel cells produce heat as well as electric power. Thus,stacked cells need to be provided with a cooling plate every a few unitcells in order to keep cell temperature constant.

[0003] There is a need to humidify the fuel gas and the oxidant gas.Hence, polymer electrolyte membranes used in polymer electrolyte fuelcells need to be moistened sufficiently with water. If the celltemperature is too high, saturated vapor pressure increases and thewater content in the polymer electrolyte membrane therefore decreases,thereby deteriorating cell performance. If the cell temperature is toolow, due to generation of water on the oxidant gas side by cellreactions, condensation of water vapor hinders sufficient permeation ofthe oxidant gas, thereby impairing cell performance. Thus, thetemperature of the fuel cell needs to be maintained within an optimaltemperature range.

[0004] A stack comprising a large number of stacked unit cells is calleda “fuel cell stack.” It comprises membrane-electrode assemblies andseparator plates having gas flow channels formed in their surfaces. Themembrane-electrode assemblies and separator plates are stackedalternately. The stack includes a current collector plate for collectinggenerated power and an insulating plate disposed on each end which aresandwiched by end plates.

[0005] Each unit cell is cooled by a coolant flowing inside theseparator plate so that the cell is maintained at suitable temperatures.However, unit cells close to the end plates tend to have lower celltemperatures in comparison with the cells in the middle of the stackbecause of heat dissipation that takes place due to the temperaturedifference between the cells and the outside air.

[0006] When a fuel cell is not generating power, no heat is generated.The cell temperature therefore in the unit cells located close to theend plates is at or close to outside temperature which is substantiallylower than the operating temperature of the cell. In such a state, ifhumidified fuel and oxidant gases are introduced in order to start powergeneration, condensation is likely to occur in the gas flow channels, inparticular in those cells furthest from the inlet of the oxidant gas tothe fuel cell. The occurrence of condensation hinders the respectivegases from permeating into the cells, possibly causing a phenomenon ofvoltage instability during power generation.

[0007] Also, when the fuel cell is controlled such that the amount ofpower generated is lower than the rated output, less heat is evolved bycell reactions. Hence, similar condensation is likely to occur in thegas flow channels in the unit cells located close to the end plates. Thetemperature is also low in cells located at the central portion of thecell stack. Thus, there is a possibility, although not so large incomparison with the unit cells located close to the end plates, ofoutput instability in the central cells.

[0008] Thus, it is necessary to control temperature such that the outputof the unit cells close to the end plates does not decrease in anyoperation state regardless of the amount of power generated.

[0009] Such voltage instability of the unit cells located close to theend plates due to condensation could be eliminated by constantlycirculating the coolant at high temperature. However, this method causesthe temperature of the coolant to rise unnecessarily in the case wherethe fuel cell generates sufficiently large amounts of power, andtherefore, large amounts of heat, so that the water content in thepolymer electrolyte membrane decreases. This impairs the powergenerating capacity of the cell because when the temperature of thecoolant is set high in an attempt to avoid the above-mentioned outputinstability caused by condensation and the amount of power generation israised thereafter, the temperature of the unit cells in the center ofthe stack becomes too high due to heat evolution by cell reactions, sothat the output of these cells decreases.

SUMMARY OF THE INVENTION

[0010] A polymer electrolyte fuel cell in accordance with the presentinvention comprises: (a) a cell stack in which unit cells each comprisea polymer electrolyte membrane sandwiched between an anode and acathode; (b) multiple unit cells stacked with each cell separated by aconductive separator plate; (c) a pair of current collector plates and apair of end plates, both of which sandwich the cell stack; (d) supplyand discharge manifolds for a fuel gas and an oxidant gas through whichthe fuel gas and the oxidant gas are supplied and discharged to and fromthe anode and the cathode of the cell stack, respectively; (e) a coolantflow channel formed in a part of the conductive separator plates; and(f) a coolant inlet and a coolant outlet for circulating a coolantthrough the coolant flow channel, wherein the conductive separator platebetween at least one of the end plates and the unit cell located closestto the one of the end plates has no coolant flow channel therein.

[0011] The conductive separator plate between the end plate which isfurthest from an oxidant gas inlet of the fuel cell and the unit celllocated closest to the end plate preferably has no coolant flow channeltherein. The conductive separator plate having no coolant flow channeltherein is preferably a separator plate which comes in contact with thecathode.

[0012] An object of the present invention is to provide a polymerelectrolyte fuel cell capable of highly efficient power generation byreducing the imbalance of output caused by the temperature differencebetween the end cells closest to the end plates and the cells located atthe center of the cell stack.

[0013] In order to achieve the above-mentioned object, the presentinvention provides a fuel cell and a method of operation capable ofeliminating overcooling of the cells located at the ends.

[0014] The present invention also provides a means for eliminatingcondensed water generated by overcooling of the cells located at theends or clogging of the gas flow channels caused thereby.

[0015] The present invention provides a method for using the polymerelectrolyte fuel cell in which the temperature of the coolant introducedinto the fuel cell is changed depending on the amount of powergeneration of the fuel cell.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a front view illustrating a polymer electrolyte fuelcell stack of Embodiment 1 of the present invention.

[0017]FIG. 2 is a front view illustrating a polymer electrolyte fuelcell stack of Embodiment 2 of the present invention.

[0018]FIG. 3 is a front view illustrating a conventional polymerelectrolyte fuel cell stack.

[0019]FIG. 4 is a plot indicating temperature characteristics of unitcells of a polymer electrolyte fuel cell stack in accordance with thepresent invention.

[0020]FIG. 5 is a plot indicating voltage characteristics of the sameunit cells of the fuel cell stack.

[0021]FIG. 6 is a plot indicating temperature characteristics of unitcells of a conventional polymer electrolyte fuel cell stack.

[0022]FIG. 7 is a plot indicating voltage characteristics of the sameunit cells of the fuel cell stack.

[0023]FIG. 8 is a plot indicating temperature characteristics of unitcells of another conventional polymer electrolyte fuel cell stack.

[0024]FIG. 9 is a plot indicating temperature characteristics of thesame unit cells of the fuel cell stack.

[0025]FIG. 10 is a diagram illustrating the structure of a fuel cellsystem in Embodiment 3 of the present invention.

[0026]FIG. 11 is a diagram illustrating the structure of a fuel cellsystem in Embodiment 4 of the present invention.

[0027]FIG. 12 is a plot indicating cell temperature characteristics of afuel cell system in Comparative Example 2.

[0028]FIG. 13 is a plot indicating cell voltage characteristics of thefuel cell system in Comparative Example 2.

[0029]FIG. 14 is a plot indicating cell temperature characteristics of afuel cell system in Example 2.

[0030]FIG. 15 is a plot indicating cell voltage characteristics of thefuel cell system in Example 2.

[0031]FIG. 16 is a plot indicating cell temperature characteristics of afuel cell system in Comparative Example 3.

[0032]FIG. 17 is a plot indicating cell voltage characteristics of thefuel cell system in Comparative Example 3.

[0033]FIG. 18 is a plot indicating cell temperature characteristics of afuel cell system in Example 4.

[0034]FIG. 19 is a plot indicating cell voltage characteristics of thefuel cell system in Example 4.

[0035]FIG. 20 is a diagram illustrating the structure of a fuel cellsystem in Embodiment 5 of the present invention.

[0036]FIG. 21 is a front view of the cathode side of a separator plateof the fuel cell in Embodiment 5 of the present invention.

[0037]FIG. 22 is a diagram illustrating the structure of a fuel cellsystem in Embodiment 6 of the present invention.

[0038]FIG. 23 is a diagram illustrating the structure of a fuel cellsystem in Embodiment 7 of the present invention.

[0039]FIG. 24 is a plot indicating cell behavior and pressure lossbehavior of a fuel cell system in Example 4.

[0040]FIG. 25 is a plot indicating cell behavior and pressure lossbehavior of a fuel cell system in Example 5.

[0041]FIG. 26 is a plot indicating cell behavior and pressure lossbehavior of a fuel cell system in Example 6.

[0042]FIG. 27 is a top view of a fuel cell in Embodiment 8 of thepresent invention.

[0043]FIG. 28 is a front view of the fuel cell in Embodiment 8 of thepresent invention.

[0044]FIG. 29 is a cross sectional view cut of the fuel cell inEmbodiment 8 of the present invention along line X-X of FIG. 27.

[0045]FIG. 30 is a front view of the cathode side of a separator plateof the fuel cell in Embodiment 8 of the present invention.

[0046]FIG. 31 is a front view of the anode side of the separator plateof the fuel cell in Embodiment 8 of the present invention.

[0047]FIG. 32 is a longitudinal sectional view of a fuel cell inEmbodiment 9 of the present invention.

[0048]FIG. 33 is a longitudinal sectional view of a fuel cell inEmbodiment 10 of the present invention.

[0049]FIG. 34 is a plot indicating change with time in voltage ofcertain unit cells of a cell in Embodiment 8 of the present invention.

[0050]FIG. 35 is a plot indicating change with time in voltage ofcertain unit cells of a cell in a comparative example.

DETAILED DESCRIPTION OF THE INVENTION

[0051] According to the present invention, a cell stack comprising unitcells separated by conductive separator plates includes at least oneconductive separator plate having no coolant flow channel therein thatis inserted between at least one end plate and the unit cell locatedclosest to the one end plate, so that overcooling of the unit cell orcells located at the end of the cell stack is prevented. Accordingly,the present invention provides a polymer electrolyte fuel cell capableof higher efficient power generation by stabilizing the output of theend cell while sufficiently cooling the cells other than the end cell.

[0052] In a preferred embodiment of the present invention, the polymerelectrolyte fuel cell further comprises a coolant temperature adjustingmeans for adjusting the temperature on the coolant inlet side, atemperature measuring means for measuring the temperature of the coolantand a temperature controlling means for controlling the coolanttemperature adjusting means on the basis of temperature information fromthe temperature measuring means. In this embodiment, it is furtherpreferable to include a second temperature measuring means for measuringthe temperature of the cell stack, wherein the temperature controllingmeans controls the coolant temperature adjusting means on the basis oftemperature information from the temperature controlling means and thesecond temperature measuring means.

[0053] In another preferred embodiment of the present invention, thepolymer electrolyte fuel cell further includes a valve installed in anexhaust path of the oxidant gas and a valve controlling means thatcloses the valve when output voltage of the cell becomes lower than apredetermined value and opens the valve after a predetermined period oftime.

[0054] In another preferred embodiment of the present invention, thepolymer electrolyte fuel cell further includes a valve installed in anexhaust path of the oxidant gas and a valve controlling means thatcloses the valve when output voltage of the cell becomes lower than afirst predetermined value and opens the valve when output voltage of thecell becomes lower than a second predetermined value which is lower thanthe first predetermined value.

[0055] In another preferred embodiment of the present invention, thepolymer electrolyte fuel cell has a mist discharge groove communicatingwith the supply and discharge manifolds for the fuel gas or oxidant gason the current collector plates in their surfaces contacting theconductive separator plates or on the conductive separator platescontacting the current collector plates in their surfaces contacting thecurrent collector plates.

[0056] It is preferred that portions of the mist discharge grooveconnecting with the supply and discharge manifolds for the fuel gas oroxidant gas be unevenly located to the lower side with respect to thedirection of gravity in which the cell stack is installed.

[0057] A method of operation of the fuel cell in accordance with thepresent invention is characterized in that the temperature of thecoolant introduced into the fuel cell is changed depending of the amountof power generated by the fuel cell. When increasing the amount of powergenerated, the temperature of the coolant is decreased continuously orin stages. When decreasing the amount of power generated by the fuelcell, the temperature of the coolant is increased continuously or instages.

[0058] The present invention provides a method of operation of thepolymer electrolyte fuel cell in which the temperature of the coolantintroduced into the fuel cell is changed depending on the temperaturesof the separator plates, the current collector plates, or the end platesof the fuel cell.

[0059] The present invention further provides for a method of operationof the polymer electrolyte fuel cell in which an exhaust path for theoxidant gas is closed and opened when output voltage of the cell becomeslower than a predetermined value in order to promote discharge of watercontent in the exhaust path.

[0060] In the following, embodiments of the present invention will bedescribed with reference to drawings.

EMBODIMENT 1

[0061]FIG. 1 illustrates a polymer electrolyte fuel cell in thisembodiment.

[0062] A fuel cell stack 10 comprises (a) membrane-electrode assemblies11, each of which constitutes a unit cell composed of a polymerelectrolyte membrane and an anode and a cathode sandwiching themembrane, and (b) conductive separator plates 12 having a coolant flowchannel 23 therein. The membrane-electrode assemblies and the conductiveseparator plates are stacked alternately to form a cell stack. On eachside of the cell stack are a membrane-electrode assembly 31, which isthe same as the above-described membrane-electrode assembly, and aconductive separator plate 32 having no coolant flow path therein. Apair of current collector plates 13, a pair of insulating plates 14, anda pair of end plates 15 are disposed outside at the ends of the stack.The resultant stack is clamped by clamping members that are not shown.

[0063] A coolant for cooling the cells flows from inlet 20 formed in oneof the end plates through an inlet-side manifold 21 that is provided soas to communicate with the insulating plate 14, current collector plate13, membrane-electrode assemblies 11 and 31, and separator plates 12 and32, into flow channels 23 formed inside the separator plates 12 to coolneighboring cells. The coolant flows through an outlet-side manifold 22and is discharged from an outlet 24 to outside of the fuel cell stack10. The coolant flow channels 23 are formed inside the separator plates12 and are almost parallel to the surfaces of the separator plates. Thecoolant flowing in channels 23 keeps the cell temperature constant.Although not shown, the fuel cell stack 10 includes inlet-side andoutlet-side manifolds for supplying each of a fuel gas and an oxidantgas to each of the unit cells. The gas flow channels communicating withthese manifolds are formed in the anode- and cathode-contacting surfacesof the separator plates. An inlet 40 and an outlet 41 for the oxidantgas are formed, for example, in the end plate 15 on the right side ofFIG. 1. Also, an inlet and an outlet for the fuel gas are formed in theend plate on the left side of the figure.

[0064] As described, this embodiment is a preferable example in whichthe separator plate 32 having therein no coolant flow channel isdisposed between the end plate on each side and the membrane-electrodeassembly 31 closest to the end plate. In this example, all the separatorplates 12 have the coolant flow channel therein, but it is also possiblethat only a part of the separator plates 12 has the coolant flowchannel. Usually, the separator plates having the coolant flow channelare arranged regularly, for example, every two cells.

EMBODIMENT 2

[0065]FIG. 2 illustrates a polymer electrolyte fuel cell in thisembodiment. In this embodiment, two conductive separator plates 32,which have no coolant flow channels, are adjacent to the end plate thatis furthest from an oxidant gas inlet 40 and which has no coolant flowchannel therein. The cell is sandwiched between the separator plates 32and is not directly cooled on both the anode and cathode sides by acoolant. The cathode side of the cell sandwiched between the separatorplate 32 and a separator plate 12 is not directly cooled by the coolant.

[0066] The above-described embodiment illustrated in FIG. 2 is not to beconstrued as limiting the number of cells stacked in the fuel cell ofthe present invention, the number of separator plates having the coolantflow channel arranged, the arrangement pattern or the like. Also, thecoolant flow channels do not need to be strictly parallel to thesurfaces of the separator plates. With respect to the number ofseparator plates 32 having no coolant circulating flow channel therein,while FIG. 2 illustrates two, the number may be more than two. However,it is desirable to set the number in consideration of the flow rate ofthe coolant, the temperature of the coolant, the current density duringpower generation by the stack, the amount of heat dissipation from theend plates and the like such that the temperatures of the inner cellsand the cells close to the end plates become as equal as possible.

[0067] Also, the above-described embodiment is not to be construed aslimiting the introducing direction of the coolant. The insulating plate14 can be integrated with the end plate by imparting an insulatingproperty to part of the end plate. As the coolant, insulating media suchas water, Florinato (3M of the U.S.), a mixture including antifreezesuch as ethylene glycol, and the like, maybe used.

COMPARATIVE EXAMPLE 1

[0068]FIG. 3 illustrates the structure of a conventional polymerelectrolyte fuel cell.

[0069] Fuel cell stack 10 comprises membrane-electrode assemblies 11,each of which constitutes a unit cell composed of a polymer electrolytemembrane, and an anode and a cathode sandwiching the membrane, andconductive separator plates 12 having a coolant flow channel 23 therein.The membrane-electrode assemblies or cells 11 and separator plates 12are stacked alternately. On both sides of this stack, a pair of currentcollector plates 13, a pair of insulating plates 14, and a pair of endplates 15 are disposed.

[0070]FIG. 6 shows one example of the result of cell temperaturemeasurement during power generation of a polymer electrolyte fuel cellin the conventional polymer electrolytes fuel cell. The cell stack iscomposed of 81 separator plates and 80 cells. All of the separatorplates have a coolant flow channel therein. The temperature of each cellwas measured by a thermocouple embedded in the separator plates thatsupplied an oxidant gas to the cells tested. B1 is a plot of time versesthe temperature of the cell closest to one end plate, and B3 is a plotof time verses the temperature of the third cell from the one end plate.B0 is the target value of cell temperature.

[0071]FIG. 6 indicates that the temperature of the cell closest to theend plate is lower than the temperature target value because the fuelcell stack is overcooled due to the influence of heat dissipation fromthe end plate side.

[0072]FIG. 7 is a plot indicating the output voltages of the unit cellsduring power generation of the conventional polymer electrolyte fuelcell stack when the temperatures shown in FIG. 6 were measured. In FIG.7, B1 and B3 represent time verses the output voltage of the cellclosest to the end plate and time verses the output voltage of the thirdcell from the end plate, respectively. As shown by B1, voltagefluctuations and a decrease in voltage were observed in the cell closestto the end plate.

[0073] The behavior of a conventional polymer electrolyte fuel cellstack during power generation will be described with reference to FIG.8, which shows another comparative example of the result of celltemperature measurement during power generation of a conventionalpolymer electrolyte fuel cell. In this example, by setting thetemperature of the coolant high, the temperature of the cell closest tothe end plate approached the target temperature. C1 is a plot of timeverses the temperature of the cell closest to one end plate, and C3 is aplot of time verses the temperature of the third cell from the one endplate. C0 is the target value of cell temperature.

[0074]FIG. 9 is a plot showing the output voltages of the unit cellsduring power generation of the polymer electrolyte fuel cell when thetemperatures shown in FIG. 8 were measured. In FIG. 9, C1 and C3represent time verses the output voltage of the cell closest to the endplate and time verses the output voltage of the third cell from the endplate, respectively. As shown by C1, the voltage fluctuations anddecrease were eliminated in the cell closest to the end plate, but theoutput voltage of the cell located inside the stack was lower as shownby C3.

EXAMPLE 1

[0075] This example describes the behavior of a polymer electrolyte fuelcell of the present invention during power generation. The cell stackused in this example is the same as the cell stack of ComparativeExample 1, except that only the separator plates closest to the endplates on both sides have no coolant flow channel as illustrated in FIG.1.

[0076]FIG. 4 is a plot of the results of cell temperature measurementduring power generation of a polymer electrolyte fuel cell according tothe present invention. A1 represents time verses the temperature of thecell closest to one end plate, and A3 represents time verses thetemperature of the third cell from the one end plate. A0 represents thetarget value of cell temperature. The plot shows that the temperature ofthe cell closest to the end plate is maintained close to the temperaturetarget value because of the balance between heat dissipation from theend plate side and heat evolution caused by power generation of thecell.

[0077]FIG. 5 is a plot showing the output voltages of the unit cellsduring power generation of the polymer electrolyte fuel cell accordingto the present invention. In FIG. 5, A1 and A3 represent time verses theoutput voltage of the cell closest to the end plate and time verses theoutput voltage of the third cell from the end plate, respectively. Thevoltage fluctuations and decrease of the cell closest to the end plate,which were observed in the conventional polymer electrolyte fuel cellstack, were not observed.

[0078] As is clear from the foregoing examples, the stack structure ofthe present invention can avoid the voltage decrease of the cellsclosest to the end plates without affecting the output of the othercells.

EMBODIMENT 3

[0079] Embodiments 3 and 4 will describe a method of using the fuel cellsystem in which the temperature of a coolant introduced into a fuel cellis varied depending on the amount of power generation of the fuel cell.The fuel cells used in this embodiment preferably comprise separatorplates having such a structure as described in Embodiments 1 and 2,i.e., the structure of coolant flow channel.

[0080] The structure of a fuel cell system in this embodiment isillustrated in FIG. 10. A fuel cell stack 10 is the same as that of theabove-described Embodiment 1. The fuel cell stack 10 has an oxidant gasinlet 40 and a fuel gas inlet 45 formed on the right end plate side.Coolant pipes 26 and 28 are connected to a coolant inlet 20 and acoolant outlet 24, respectively. The coolant pipe 28 branches into 28 aand 28 b. The pipe 28 b comprises a heat exchanger 56. The pipes 28 aand 28 b are connected to the pipe 26 via a temperature adjusting means50. The pipe 26 is connected to a temperature measuring means 52. Thepipe 28 b heats, for example, water from a hot water supply system bymeans of the heat exchanger 56. The water to be heated enters throughinlet 57 and exits through outlet 58. The temperature adjusting means 50adjusts the temperature of a coolant flowing through the pipe 26 bysignals sent from a temperature controlling means 51 based on thetemperature information from the temperature measuring means 52. Thetemperature adjusting means 50 adjusts the temperature of the coolantintroduced into the inlet 20 of the stack 10 by adjusting the mixingratio of the heated coolant coming from the pipe 28 a and the coolantcooled by the heat exchanger 56.

EMBODIMENT 4

[0081]FIG. 11 illustrates the structure of a fuel cell system in thisembodiment. A stack temperature measuring means 54 is incorporated in aseparator plate 12 at the right end of a fuel cell stack 10. Atemperature controlling means 51 determines the control targettemperature of the coolant temperature based on a predeterminedfunction, the coolant temperature detected by a temperature measuringmeans 52, and the stack temperature detected by the temperaturemeasuring means 54. The temperature controlling means sends controlsignals to a coolant temperature adjusting means 50 that adjusts thecoolant temperature based on the information provided by the temperaturecontrolling means. Accordingly, the fuel cell stack 10 is maintained attemperatures appropriate for power generation.

[0082] The fuel cell systems described in the foregoing Embodiments 3and 4 are merely examples of the present invention, and theseembodiments are not to be construed as limiting the number of cellsstacked in the fuel cell stack of the present invention and the numberof the separator plates having a coolant flow channel. Also, theseembodiments are not to be construed as limiting the setting positions ofthe coolant temperature measuring means and coolant temperatureadjusting means. It is also possible to provide the coolant temperatureadjusting means on the fuel cell stack exit side of the coolant pipe.The coolant temperature adjusting means is not necessarily unitary.Also, the heating means and cooling means may be provided separately.Further, the insulating plate can be integrated with the end plate byimparting an insulating property to part of the end plate. The coolanttemperature does not need to be raised in one operation as in theforegoing embodiments, and it may be done in a few operations. Thetiming for raising the coolant temperature is not necessarily concurrentwith the start of power generation as in these embodiments. There may bea time lag of several minutes after starting power generation before thecoolant temperature is raised.

COMPARATIVE EXAMPLE 2

[0083] The changes of the temperature and cell voltage when increasingthe power generation capacity of a fuel cell system of this comparativeexample will be described.

[0084] The cell stack is the same as that of Example 1. The temperaturecontrolling means is set such that the temperature of the coolantflowing into the fuel cell stack is 76.3° C.

[0085]FIG. 12 is a plot of time versus the temperature of separatorplates incorporated into the fuel cell stack 10 before and after anincrease in power generation capacity of the fuel cell system of thiscomparative example.

[0086] In FIG. 12, B101 represents time verses the temperature of theseparator plate closest to one end plate, B110 represents time versesthe temperature of the separator plate stacked at the central portion ofthe fuel cell stack, and B100 represents the temperature of the coolantat the location where the coolant flows into the fuel cell stack. FIG.12 shows that the separator plate stacked at the central portion of thefuel cell stack exhibits a considerable temperature rise with increasingpower generation capacity, unlike the separator plate closest to the endplate. On the other hand, the temperature of the separator plate closestto the end plate does not rise.

[0087]FIG. 13 is a plot of the output voltage of unit cells incorporatedinto the above-described fuel cell stack 10. The output voltage B110 ofthe unit cell stacked at the central portion of the fuel cell stackdrops due to the increase in current density caused by the increasedpower generation capacity. Thereafter, the voltage gradually decreasesdue to the rise of cell temperature and therefore the reduction in watercontent of the polymer electrolyte membrane.

EXAMPLE 2

[0088] The method of operation of a system having the structure of FIG.10 incorporating therein the same cell stack as that of Example 1 willbe described. In this example, setting was made on the temperaturecontrolling means so that the temperature of the coolant at the locationwhere the coolant was introduced into the fuel cell stack was changedfrom 76.3° to 75.0° C. upon increasing the amount of power generated.

[0089]FIG. 14 is a plot of time versus the temperature of separatorplates incorporated into the fuel cell stack 10 before and after anincrease in power generation capacity of the fuel cell system. A101represents time verses the temperature of the separator plate closest toone end plate, A110 represents time verses the temperature of theseparator plate at the central portion of the fuel cell stack, and A100represents the temperature of the coolant at the location where thecoolant flows into the fuel cell stack. FIG. 14 shows that thetemperature of each cell is, unlike Comparative Example 2, maintained atan appropriate temperature because of the decrease in coolanttemperature upon the increase in power generation capacity.

[0090]FIG. 15 is a plot of the output voltage of unit cells incorporatedinto the fuel cell stack 10 from at the time of non-power generationduring operation of the fuel cell system until after a lapse of acertain time from the start of power generation. The drop in outputvoltage A110 of the unit cell at the central portion of the fuel cellstack is small when compared to B110 of FIG. 13. Presumably, this isbecause the rise in cell temperature could be suppressed by the effectsof the present invention.

COMPARATIVE EXAMPLE 3

[0091] The changes of the temperature and cell voltage when decreasingthe power generation capacity of the same fuel cell system as that ofComparative Example 2 will be described. The temperature controllingmeans is set such that the temperature of the coolant flowing into thefuel cell stack is 75.0° C.

[0092]FIG. 16 is a plot of time versus the temperature of the separatorplates incorporated into the fuel cell stack 10 before and after adecrease in power generation capacity. B201 represents the temperatureof the separator plate closest to one end plate, B210 represents thetemperature of the separator plate stacked at the central portion of thefuel cell stack, and B200 represents the temperature of the coolant atthe location where the coolant flows into the fuel cell stack. FIG. 16shows that the separator plate closest to the end plate exhibits aconsiderable temperature decrease with decreasing power generationcapacity, unlike the separator plate at the central portion of the fuelcell stack. The temperature of the separator plate stacked at thecentral portion also decreases with decreasing power generationcapacity.

[0093]FIG. 17 is a plot of time versus the output voltage of unit cellsincorporated into the fuel cell stack 10 before and after the decreasein power generation capacity. The output voltage B201 of the unit cellclosest to the end plate is unstable because condensation occurred inthe oxidant gas flow channel due to the temperature decrease caused bythe decreased power generation capacity.

EXAMPLE 3

[0094] The same fuel cell system as that of Example 2 was used. Thetemperature controlling means was set so that the temperature of thecoolant at the location where the coolant was introduced into the fuelcell stack was changed from 75.0° to 76.3° C. upon decreasing the amountof power generated by the fuel cell.

[0095]FIG. 18 is a plot of time versus the temperature of the separatorplates incorporated into the fuel cell stack 10 before and after adecrease in power generation capacity of the fuel cell system. A201represents the temperature of the separator plate closest to one endplate, A210 represents the temperature of the separator plate at thecentral portion of the fuel cell stack, and A200 represents thetemperature of the coolant at the location where the coolant flows intothe fuel cell stack. FIG. 18 indicates that the temperature of each cellis, unlike Comparative Example 3, maintained at an appropriatetemperature because of the increase in coolant temperature upon thedecrease in power generation capacity.

[0096]FIG. 19 is a plot of time versus the output voltage of unit cellsincorporated into the fuel cell stack 10 from at the time of non-powergeneration during operation of the fuel cell system until after a lapseof a certain time from the start of power generation. The output voltageA201 of the unit cell closest to the end plate is stable. Presumably,this is because of the effects of the present invention. No condensationoccurred in the oxidant gas flow channel in the unit cell closest to theend plate. The power output A210 of the unit cell at the center exhibitsa slightly lower value in comparison with B201 of FIG. 17. This isbecause the rise of the coolant temperature induced a decrease in watercontent of the polymer electrolyte membrane in the cell. However, thiscan prevent the large output drop caused by condensation in the oxidantgas flow channel. Further, the degree of the output drop can be reducedsufficiently by appropriately adjusting how much the coolant temperatureis increased upon the decrease in power generation capacity.

[0097] In increasing or decreasing the power generation capacity, it isdifficult to unconditionally specify the degree the coolant temperaturewill be raised or lowered since the temperature varies depending on thespecifications of each system such as the amount of power generated, theflow rate of the coolant and the area of the cell. There is no problemif settings are made such that the temperature difference between beforeand after the change of the power generation capacity of the unit cellclosest to the end plate is within 2° C., and more preferably withinapproximately 1° C.

EMBODIMENT 5

[0098] Embodiments 5, 6 and 7 below are directed to a fuel cell and amethod of use capable of eliminating water clogging occurring inside acell stack in a simple and reliable manner. The fuel cells used in theseembodiments preferably comprise separator plates having such a structureas described in Embodiments 1 and 2.

[0099]FIG. 20 illustrates the structure of a polymer electrolyte fuelcell of Embodiment 5.

[0100] A fuel cell stack 60 has a structure in which MEAs 61 andseparator plates 62 are stacked alternately. This stack further includesa current collector plate 63 and an insulating plate 64 disposed on eachend of the stack. The stack, current collector place 63 and insulatingplate 64 are sandwiched by end plates 65. The resultant stack is clampedby clamping members (not shown) with a predetermined load. The endplates 65 are provided with an oxidant gas inlet portion 80 and a fuelgas inlet portion 85, to which an oxidant gas and a fuel gas aresupplied respectively from a gas supply apparatus (not shown). The fuelcell further includes a gas outlet portion 86 and an oxidant gas outletportion 81 from which the fuel gas and the oxidant gas are discharged,respectively.

[0101] As illustrated in FIG. 21, the separator plate 62 comprises anoxidant gas inlet-side manifold aperture 70 and an oxidant gasoutlet-side manifold aperture 71. The separator plate further includes afuel gas inlet-side manifold aperture 75, a fuel gas outlet-sidemanifold aperture 76, and a gas flow channel 74 communicating with themanifold apertures 70 and 71 formed on the cathode-facing side thereof.On the other side of the separator plate 62 is formed a fuel gas flowchannel. The separator plate having a coolant flow channel used is, forexample, a composite separator plate obtained by bonding a cathode-sideseparator plate, which has an oxidant gas flow channel on one side and acoolant flow channel on the other side, and an anode-side separatorplate, which has a fuel gas flow channel on one side and a coolant flowchannel on the other side, together in such a manner that their sideshaving the coolant flow channel face each other. In FIG. 21, coolantmanifold apertures are omitted.

[0102] The oxidant gas outlet portion 81 is provided with a valve 91.However, if there is an exhaust pipe connected to the oxidant gas outletportion 81, the valve 91 may be installed within the exhaust pipe. Thevalve 91 may be installed at any location on the exhaust path of theoxidant gas discharged outside from the outlet-side manifold. Also, thepresent invention is applicable not only to the above-described internalmanifold type having manifold apertures formed inside the separatorplates, but also to the external manifold type having manifold aperturesformed outside the separator plates.

[0103] When this polymer electrolyte fuel cell is operated for a longtime, water clogs the oxidant gas flow channels and diffusion layerscausing the cell voltage to lower gradually. By closing the valve 91 fora predetermined time and opening the valve again, water can bedischarged with the oxidant gas at a high flow rate from the outlet sideof the oxidant gas. The predetermined time of valve closing ispreferably 1 to 20 seconds.

EXAMPLE 4

[0104] A cathode catalyst was prepared by placing platinum particleshaving an average particle size of approximately 30 Å in a weight ratioof 50:50 on conductive carbon particles having an average primaryparticle size of 30 nm, Ketjen Black EC (manufactured by AKZO ChemieCompany of the Nederlands). An anode catalyst was prepared by placingplatinum particles and ruthenium particles, each having an averageparticle size of approximately 30 Å, in a weight ratio of 25:25:50, onKetjen Black EC carbon particles having an average primary particle sizeof 30 nm. A dispersion of each of these catalysts in isopropanol wasmixed with a dispersion of perfluorocarbon sulfonic acid powder in ethylalcohol to form a paste. The paste containing the cathode catalyst wasapplied by screen printing onto one side of a 250 μm thick carbon fibernonwoven fabric to form a cathode catalyst layer. The paste containingthe anode catalyst was applied by screen printing onto one side ofanother 250 μm thick carbon fiber nonwoven fabric to form an anodecatalyst layer. In each of the catalyst layers, the content of catalystmetal was 0.5 mg/cm², and the content of perfluorocarbon sulfonic acidwas 1.2 mg/cm².

[0105] Subsequently, the nonwoven fabric containing the anode catalystlayer and the nonwoven fabric containing the cathode catalyst layer werebonded by hot pressing to the center part of a hydrogen-ion conductivepolymer electrolyte membrane such that the anode catalyst layer bondedto one side of the membrane and the cathode catalyst layer bonded to theopposite side of the membrane. The conductive polymer electrolytemembrane has an area slightly larger than that of the anode and cathodein such a manner that the catalyst layers sufficiently adhered to theelectrolyte membrane. The hydrogen-ion conductive polymer electrolytewas a thin film of perfluorocarbon sulfonic acid (Nafion 112manufactured by E. I. Du Pont de Nemours & Co. Inc. of the UnitedState). Further, gaskets, punched out into the same shape as that of theseparator plate, were bonded by hot pressing to the electrolyte membraneso as to sandwich the electrolyte membrane which was positioned on theouter periphery of the electrodes, whereby an MEA (membrane-electrodeassembly) was produced.

[0106] The operation of the polymer electrolyte fuel cell of thisexample is described with reference to FIG. 24. The conditions of theexperiment included the use of a simulated reformed gas (80% hydrogen byvolume, 20% carbon dioxide by volume, and 50 ppm carbon monoxide) as afuel gas and air as an oxidant gas. The fuel gas, humidified and heatedto have a dew point of 75° C., and the air, humidified and heated tohave a dew point of 50° C., were supplied to the fuel cell.Characteristic testing was performed under the conditions of a fuelutilization rate of 80%, oxygen utilization rate of 50%, celltemperature of 75° C. and current density of 0.3 A/cm². The fuel cellstack has the same structure as that as described in Example 1.

[0107]FIG. 24 is a graph showing the changes in cell voltage andcathode-side pressure loss upon opening and closing the valve 91 duringthe operation of the fuel cell under the above-mentioned conditions.When the valve 91 which had been open was closed, the pressure lossincreased as expected naturally, thereby making supply of the oxidantgas difficult, so that the cell voltage decreased gradually. Then, whenthe valve 91 was opened again approximately 8 seconds after the closingof the valve 91, the oxidant gas was discharged at a high flow rate fromthe cathode side, and at the same time, the water accumulated in thecell was discharged. As a result, it was found that the cell voltagemade an instant recovery and exceeded the voltage value before theclosing of the valve 91. This demonstrates that the cell voltage loweredby water clogging or the like can be restored in a short time and in areliable manner by closing the valve installed on the exhaust path ofthe oxidant gas for a predetermined time and opening the valve again.

[0108] The predetermined time of valve closing is preferably 1 to 20seconds. If the time is shorter than one second, cell voltage recoverycannot be obtained. If it is longer than 20 seconds, a problem of thecell voltage drop becoming larger arises. The cell voltage monitored inthis embodiment is that of the unit cell.

EMBODIMENT 6

[0109] The cell structure of this embodiment is illustrated in FIG. 22.It is the same as that of Embodiment 5, except for the use of anelectromagnetic valve 92 in place of the valve 91 and the addition of anelectromagnetic valve open/close controlling means 93 for controllingthe opening and closing of the electromagnetic valve. Theelectromagnetic valve open/close controlling means 93 was furnished witha controlling function by means of a computer. However, it may befurnished with a controlling function by means of an analogue circuitcomprising a comparator or the like.

[0110] With this fuel cell, a series of operations of closing theelectromagnetic valve for a predetermined time and opening theelectromagnetic valve again can be performed periodically andautomatically to prevent the cell voltage from lowering too much. It istherefore possible to suppress performance deterioration occurring in acontinuous operation over an extended period of time.

EMBODIMENT 7

[0111] The cell structure of this embodiment is illustrated in FIG. 23.It is the same as that of Embodiment 6, except that a voltage detectingmeans 94 for detecting cell voltage is included such that when thevoltage detected by this voltage detecting means becomes lower than apredetermined threshold value, an electromagnetic valve open/closecontrolling means 93 closes a valve 92 for a predetermined time. Acommon voltage measuring terminal was used for the voltage detectingmeans 94, but an AD converter or the like may also be used.

[0112] With regard to the operation of the electromagnetic valveopen/close controlling means 93, there are two methods. The first methodcloses the valve when the cell voltage becomes lower than the valveclose threshold value and opens the valve after a lapse of apredetermined time. The second method closes the valve when the cellvoltage becomes lower than the valve close threshold value and opens thevalve when the cell voltage becomes lower than the valve open thresholdvalue. The valve close threshold value is preferably in a range of 0.55to 0.65 V/cell. The valve open threshold value is preferably 0.3 V to0.5 V/cell.

EXAMPLE 5

[0113]FIG. 25 shows the behavior of cell voltage when the fuel cell wasoperated under the same conditions as those of Example 4, except for theuse of the first method in which the valve operating threshold value wasset to 0.6 V/cell. As the time elapsed, water gradually clogged thecell, so that the cell voltage detected by the voltage detecting means94 was lowered. When the cell voltage reached the valve close thresholdvalue 0.6 V/cell or lower, the electromagnetic valve 92 was closed bythe output of the electromagnetic valve open/close controlling means 93,and after a lapse of approximately 12 seconds, the electromagnetic valvewas opened again. As a result, water in the cell was discharged, and thecell voltage was improved in comparison with before the closing of theelectromagnetic valve. The valve close threshold value is preferably ina range of 0.55 to 0.65 V/cell. When the threshold value is lower thanthis range, sufficient elimination of water clogging becomes difficult.When it is higher, the electromagnetic valve opens and closesfrequently, which is not preferable. The closing time of theelectromagnetic valve is preferably 1 to 20 seconds in the same manneras in Example 4.

[0114] As described above, the electromagnetic valve open/closecontrolling means 93 controls the timing of closing the electromagneticvalve 91 by comparing the cell voltage monitored by the voltagedetecting means 94 with the valve close threshold value. After theclosing of the electromagnetic valve 92, it exercises control such thatthe electromagnetic valve is opened after a lapse of the predeterminedtime. Therefore, it becomes possible to automatically open and close theelectromagnetic valve with adequate timing and to ensure stableperformance even in a long-time continuous operation.

EXAMPLE 6

[0115] Next, FIG. 26 indicates the behavior of cell voltage in thesecond method. The conditions of the experiment were the same as thoseof Example 4. When the operation of the cell was continued for a longtime, water clogging gradually occurred in the cell, so that the cellvoltage was lowered. When the cell voltage detected by the voltagedetecting means 94 reached the valve close threshold value 0.6 V/cell orlower, the electromagnetic valve 92 was closed by the output of theelectromagnetic valve open/close controlling means 93. When the cellvoltage reached the valve open threshold value 0.5 V/cell or lower, theelectromagnetic valve 92 was opened again by the output of theelectromagnetic valve open/close controlling means 93. As a result,water in the cell was discharged, and the cell voltage was improved incomparison with before the closing of the electromagnetic valve 92. Thevalve close threshold value is preferably in a range of 0.55 to 0.65V/cell, as described above. The valve open threshold value is preferably0.3 to 0.5 V/cell. When the valve open threshold value is higher thanthis range, elimination of water clogging becomes insufficient and thussufficient effects of cell voltage recovery cannot be obtained. When itis lower, the cell voltage drop becomes larger.

[0116] As described above, the electromagnetic valve open/closecontrolling means 93 not only controls the timing of closing theelectromagnetic valve 92 by comparing the cell voltage monitored by thevoltage detecting means 94 with the valve close threshold value, butalso controls, after the closing of the electromagnetic valve 92, thetiming of opening the electromagnetic valve by comparing the cellvoltage monitored by the voltage detecting means 94 with the valve openthreshold value. Therefore, in comparison with the first method, it ispossible to prevent the cell voltage from dropping extremely while theelectromagnetic valve is closed. Hence it is possible to automaticallyopen and close the electromagnetic valve with more adequate timing andto ensure stable performance even in a long-time continuous operation.

[0117] In Examples 5 and 6, the valve open/close controlling means 93controlled the electromagnetic valve 92 such that it was closed when thecell voltage detected reached the valve close threshold value or lower.However, it may also control the electromagnetic valve 92 such that itis closed at intervals in which the cell voltage is expected to reachthe valve close threshold value. The voltage detecting means 94monitored the voltage value of one unit cell. However, it may alsomonitor the voltage of a cell stack or the total voltage value of aplurality of unit cells such as the voltage of every four cells.

EMBODIMENT 8

[0118] Embodiments 8 to 12 will describe a fuel cell in whichcondensation-induced mist is effectively discharged to prevent theoccurrence of uneven gas distribution to each cell due to the mist. Whena fuel cell is operated for a long period of time, condensation occursat portions close to end plates. Mist, induced by the condensation,enters gas supply channels to cause clogging thereof. Then, gasdistribution becomes uneven between the cells with a clogged gas supplychannel and the cells without clogging. The means for solving thisproblem is detailed below. The fuel cells used in these embodimentspreferably comprise separator plates having such a structure asdescribed in Embodiments 1 and 2, i.e., the structure of coolant flowchannel.

[0119]FIG. 27 is a plane view of a fuel cell in this embodiment. FIG. 28is a front view thereof. FIG. 29 is a cross-sectional view cut along theline X-X of FIG. 27.

[0120] A membrane-electrode assembly (MEA) is represented by 110. TheMEA 110 is composed of a polymer electrolyte membrane 101, a pair ofelectrodes (not shown) sandwiching the membrane, and gaskets 102 and 103sandwiching the electrolyte membrane on the periphery of the electrodes.A plurality of MEAs 110 are stacked with conductive separator plates 111interposed therebetween to form a stack of unit cells. The separatorplate 111 has a gas flow channel 114 for supplying an oxidant gas to thecathode on one side, and has a gas flow channel 115 for supplying a fuelgas to the anode on the other side. The separator plate 111 serves bothas a cathode-side separator plate and as an anode-side separator plate.In this example, separator plates 121 a and 121 b are additionallyprovided at the ends of the cell stack. Current collector plates 128 aand 128 b, insulating plates 127 a and 127 b, and end plates 129 a and129 b are provided outside. The end plates 129 a and 129 b are joined byclamping means composed of a plurality of bolts 133, nuts 134 screwed tothe ends thereof, and springs 135 to clamp the cell stack. This cell iscovered with a heat insulating material which is not shown.

[0121] The separator plates 111 and 121 a, 121 b are provided withoxidant gas manifold apertures 112 and 122 and fuel gas manifoldapertures 113 and 123, respectively The MEAs 110, current collectorplate 128 a, insulating plate 127 a and end plate 129 a comprisemanifold apertures communicating with the above-mentioned respectivemanifold apertures. The oxidant gas is introduced into these manifoldapertures from an oxidant gas inlet pipe 130 a installed to the endplate 129 a The oxidant gas is supplied from the gas flow channels 114of the separator plates 111 to the cathodes and is discharged to outsidefrom an oxidant gas discharge pipe 130 b via outlet-side manifoldapertures (not shown). Likewise, the fuel gas is introduced into themanifold apertures 113 and 123 from a fuel gas inlet pipe 131 ainstalled to the end plate 129 a, is supplied from the gas flow channels115 of the separator plates 111 to the anodes and is discharged tooutside from a fuel gas discharge pipe 131 b via outlet-side manifoldapertures (not shown).

[0122] Also, the separator plates 111 and 121 a, 121 b are provided withcooling water manifold apertures 116 and 126. The MEAs 110, currentcollector plate 128 a, insulating plate 127 a and end plate 129 acomprise manifold apertures communicating with the above-mentionedmanifold apertures. Cooling water is introduced into these manifoldapertures from a cooling water inlet pipe 132 a installed to the endplate 129 a. The cooling water flows through cooling sections forcooling predetermined unit cells and is discharged to outside from adischarge pipe 132 b via outlet-side manifold apertures. In the exampleshown in the figure, the cooling sections are omitted.

[0123] The separator plate 121 a is located between the separator plate111 at the end of the cell stack and the current collector plate 128 a.It has, on its side contacting the current collector plate 128 a, a mistdischarge groove 124 communicating with the oxidant gas manifoldapertures 122. Likewise, the separator plate 121 b is located betweenthe separator plate 111 at the end of the cell stack and the currentcollector plate 128 b. It has, on its side contacting the currentcollector plate 128 b, a mist discharge groove 125 communicating withthe fuel gas manifold apertures 123.

[0124]FIGS. 30 and 31 are front views of the separator plate seen fromthe cathode side and the anode side, respectively. It has the gas flowchannel 114 communicating with a pair of the manifold apertures 112 onits cathode side and has the gas flow channel 115 communicating with apair of the manifold apertures 113 on its anode side. The mist dischargegroove 124 of the separator plate 121 a is formed so as to communicatewith a pair of the oxidant gas manifold apertures 122 similarly to theoxidant gas flow channel 114. The mist discharge groove 125 of theseparator plate 121 b is formed so as to communicate with a pair of thefuel gas manifold apertures 123 similarly to the fuel gas flow channel115. The pattern of these grooves 124 and 125 is merely an example, andis not to be limited to the pattern as illustrated in the figure.

[0125] As described above, in this embodiment, additional separatorplates 121 a and 121 b are provided at the ends of the cell stack. Theseparator plate 121 a and the separator plate 121 b are provided withthe mist discharge groove 124 communicating with the oxidant gas supplychannel and the mist discharge groove 125 communicating with the fuelgas supply channel, respectively. The sides of the grooves contact thecurrent collector plates.

[0126] In a fuel cell having this structure, the water contained in theoxidant gas and fuel gas may condense on the current collector plates128 a and 128 b, and on the separator plates 121 a and 121 b, whichcontact the current collector plates to form mist due to heatdissipation from the end plates 129 a and 129 b. However, such mist isdischarged to the outside from the mist discharge grooves 124 and 125formed at the interface between the separator plates 121 a and 121 b andthe current collector plates via the oxidant gas discharge path and thefuel gas discharge path, respectively. Therefore, the mist is preventedfrom entering the gas flow channels 114 and 115 located adjacentthereto, so that the blockage of the gas flow channels does not occurand stable gas distribution therefore becomes possible. As described,the present invention enables stable operation of the cell andeliminates the need for excessive heat insulation structures.

EXAMPLE 7

[0127] A cathode catalyst was prepared by placing platinum particleshaving an average particle size of approximately 30 Å in a weight ratioof 25:75 on carbon fine powder having an average particle size of 30 nm(DENKA BLACK, manufactured by Denki Kagaku Kogyo K.K.). An anodecatalyst was prepared by placing platinum-ruthenium particles(platinum/ruthenium weight ratio 50:50) in a weight ratio of 50:50 onDENKA BLACK carbon fine powder having an average particle size of 30 nm.A dispersion of each of these catalyst powders in isopropanol was mixedwith a dispersion of perfluorocarbon sulfonic acid powder in ethylalcohol to form a paste. Using these pastes, a cathode catalyst layerwas formed by screen printing onto one side of a 250 μm thick carbonfiber nonwoven fabric to form a cathode. An anode catalyst layer wasformed on another 250 μm thick carbon fiber nonwoven fabric to form ananode. The content of catalyst metal in each layer was to 0.5 mg/cm²,and the content of perfluorocarbon sulfonic acid was to 1.2 mg/cm².

[0128] The cathode and the anode were bonded by hot pressing to theopposing sides of the center part of a hydrogen-ion conductive polymerelectrolyte membrane in such a manner that the printed catalyst layerswere brought in contact with the electrolyte membrane. The area of theconductive polymer electrolyte membrane was slightly larger than that ofthe cathode and anode. The hydrogen-ion conductive polymer electrolytewas a thin film of perfluorocarbon sulfonic acid (Nafion 112manufactured by E. I. Du Pont de Nemours & Co. Inc. of the United State)having a thickness of 25 μm. Further, gaskets, punched out into the sameouter shape as that of the separator plate, were bonded by hot pressingto the outer periphery of the electrodes so as to sandwich theelectrolyte membrane, whereby an MEA (membrane-electrode assembly) wasproduced.

[0129] Using 100 MEAs, a fuel cell as illustrated in FIGS. 27 to 29 wasassembled. As a comparative example, a cell without the groove 124 ofthe separator plate 121 a and the groove 125 of the separator plate 121b was also assembled.

[0130] While these cells were retained at a temperature of 75° C., asimulated reformed gas (80% hydrogen by volume, 20% carbon dioxide byvolume, and 50 ppm carbon monoxide), humidified and heated to have a dewpoint of 75° C., and air, humidified and heated to have a dew point of50° C., were supplied to the anode and the cathode, respectively. Thesecells were operated under the conditions of a fuel utilization rate of80%, oxygen utilization rate of 50%, and current density of 0.3 A/cm².The change in cell voltage with passage of time was examined on the cellat the central portion and the cells located at the ends. Thecharacteristics of the cell of this example and the characteristics ofthe cell of the comparative example are shown in FIG. 34 and FIG. 35,respectively. As is apparent from these figures, in the stacked cell ofthe comparative example, the voltages of the cells located at the endsdropped sharply after a lapse of a certain time, whereas in the cell ofthis example, no voltage drop occurred due to the effect of the mistdischarge grooves.

EMBODIMENT 9

[0131]FIG. 32 is a cross-sectional view of a fuel cell of thisembodiment cut along the counterpart of the line X-X of FIG. 27. Thebasic structure of this fuel cell is the same as that of Embodiment 8,and the same reference numerals represent the same components, asdescribed above for Embodiment 8.

[0132] In this embodiment, separator plates 111 a and 111 b at the endsof a cell stack function only as a cathode-side separator plate and ananode-side separator plate, respectively. Current collector plates 128 aand 128 b contacting these separator plates have a mist discharge groove124 communicating with oxidant gas manifold apertures, which is on theupper side of the figure. A mist discharge groove 125 communicating withfuel gas manifold apertures is on the lower side thereof.

[0133] In this fuel cell, the water mist produced between the separatorplate 111 a and the current collector plate 128 a and between thecurrent collector plate 128 b and the insulating plate 127 b isdischarged to outside from the grooves 125 of the current collectorplates 128 a and 128 b, respectively, via a fuel gas discharge path.Also, the water mist produced between the current collector plate 128 aand the insulating plate 127 a and between the separator plate 111 b andthe current collector plate 128 b is discharged to outside from grooves124 of the current collector plates 128 a and 128 b, respectively, viaan oxidant gas discharge path.

EMBODIMENT 10

[0134]FIG. 33 is a cross-sectional view of a fuel cell of thisembodiment cut along the counterpart of the line X-X of FIG. 27. Thebasic structure of this fuel cell is the same as that of Embodiment 8,and the same reference numerals represent the same components as inEmbodiment 8.

[0135] In this embodiment, the mist discharge groove is so configuredthat its portions connecting with manifold apertures are unevenlylocated to the lower side of the manifold apertures in the direction ofgravity at the installation position of the cell. That is, a groove 115a on the right side of a separator plate 111 a and a groove 125 b on theleft side of a separator plate 121 b, both of which communicate with afuel gas discharge path, are so configured that their portionsconnecting with manifold apertures are unevenly located to the lowerside of the manifold apertures in the direction of gravity. In theseparator plate 111 of FIG. 31, the fuel gas flow channel 115 is soconfigured that its portions connecting with the manifold apertures 113are almost evenly arranged with respect to the manifold apertures. Thegrooves 115 a and 125 b, which correspond to this flow channel, areunevenly located to the lower side of the manifold apertures. Also, agroove 124 a on the right side of a separator plate 121 a and a groove114 b on the left side of a separator plate 111 b, both of whichcommunicate with an oxidant gas discharge path, are so configured thattheir portions connecting with manifold apertures are unevenly locatedto the lower side of the manifold apertures in the direction of gravity.

[0136] With this structure, even if the water contained in the gasescondenses on the current collector plates and the separator platescontacting the current collector plates to form mist due to heatdissipation from the end plates, the mist collected by gravitation iseffectively discharged from the mist discharge grooves arranged on thelower side of the direction of gravity.

EMBODIMENT 11

[0137] In this embodiment, in a cell having the above-describedstructure, mist discharge grooves are provided with a hydrophiliccoating comprising a polyurethane resin (trade name M1210, manufacturedby TOAGOSEI CO., LTD) by photopolymerization. The hydrophilic coatingsurface allows the condensation-induced mist to be dischargedeffectively without causing it to build up in the mist dischargegrooves.

[0138] For the hydrophilicity treatment of the mist discharge grooves,it is also possible to use polyvinyl alcohol, polyester-type resin(manufactured by Nippon Kayaku Co., Ltd.), 2-hydroxyl acrylate resin,bovine serum albumin (BSA), polyglutamic acid, silica gel or the like.The same effects were obtained, for example, when the inner surfaces ofthe mist discharge grooves were roughened by sandblasting treatment inthe case of conductive separator plates made of a carbon-based materialand when the inner surfaces of the mist discharge grooves formed incurrent collector plates were coated with titanium and were subjected toheat treatment of 400° C. under an oxygen atmosphere for one hour toform a titanium oxide film on the surfaces thereof.

EMBODIMENT 12

[0139] In this embodiment, in a cell having the above-describedstructure, polyester woven fabric is inserted in mist discharge groovesas a water absorbing material. Since the mist produced by condensationis absorbed by the water absorbing material of the mist dischargegrooves, it is effectively collected to the grooves and discharged bythe pressure of the oxidant gas or fuel gas.

[0140] As the water absorbing material, it is also possible to usepolyethylene terephthalate, rayon, nylon, material composed mainly ofblended fiber thereof, or the like.

[0141] Also, the use of fiber or porous body of carbon, stainless steelor titanium as the water absorbing material allows reduction in contactresistance between the current collector plate and the separator plate,as well as the above-described effect of the water absorbing material,so that the effect of improving cell performance can also be obtained.

[0142] As described above, the present invention can provide a polymerelectrolyte fuel cell which exhibits stable performance by reducing thedifference in output between the cells closest to end plates and theother cells. Also, in the event of occurrence of mist in a cell stackand of water clogging due to condensation, it is possible to readilyeliminate them and restore the performance.

[0143] While this invention has been described with reference to severalpreferred embodiments, it is contemplated that various alterations andmodifications thereof will become apparent to those skilled in the artupon a reading of the preceding detailed description. It is thereforeintended that the following appended claims be interpreted as includingall such alterations and modifications as fall within the true spiritand scope of this invention.

What is claimed is:
 1. A polymer electrolyte fuel cell comprising: acell stack in which unit cells each comprising a polymer electrolytemembrane and an anode and a cathode sandwiching said electrolytemembrane are stacked with conductive separator plates; a pair of currentcollector plates and a pair of end plates, both of which sandwich saidcell stack; supply and discharge manifolds for a fuel gas and an oxidantgas, through which the fuel gas and the oxidant gas are supplied anddischarged to and from the anode and the cathode of the cell stack,respectively; a coolant flow channel formed in a part of said conductiveseparator plates; and a coolant inlet and a coolant outlet forcirculating a coolant through said coolant flow channel, wherein theconductive separator plate between at least one of the end plates andthe unit cell located closest to said one of the end plates has nocoolant flow channel therein.
 2. The polymer electrolyte fuel cell inaccordance with claim 1, wherein the conductive separator plate betweenthe end plate which is furthest from an oxidant gas inlet of said fuelcell and the unit cell located closest to the end plate has no coolantflow channel therein.
 3. The polymer electrolyte fuel cell in accordancewith claim 1, further comprising a coolant temperature adjusting meansfor adjusting the temperature on said coolant inlet side; a temperaturemeasuring means for measuring the temperature of said coolant; and atemperature controlling means for controlling said coolant temperatureadjusting means on the basis of temperature information from saidtemperature measuring means.
 4. The polymer electrolyte fuel cell inaccordance with claim 3, further comprising a second temperaturemeasuring means for measuring the temperature of the cell stack, whereinsaid temperature controlling means controls said coolant temperatureadjusting means on the basis of temperature information from saidtemperature controlling means and the second temperature measuringmeans.
 5. The polymer electrolyte fuel cell in accordance with claim 1,further comprising a valve installed on an exhaust path of said oxidantgas and a valve controlling means that closes said valve when outputvoltage of said cell becomes lower than a predetermined value and opensthe valve after a predetermined time.
 6. The polymer electrolyte fuelcell in accordance with claim 1, further comprising a valve installed onan exhaust path of said oxidant gas and a valve controlling means thatcloses said valve when output voltage of said cell becomes lower than afirst predetermined value and opens said valve when output voltage ofsaid cell becomes lower than a second predetermined value which is lowerthan the first predetermined value.
 7. The polymer electrolyte fuel cellin accordance with claim 1, wherein the conductive separator platescontacting said current collector plates have a mist discharge groovecommunicating with said supply and discharge manifolds for the fuel gasor oxidant gas in their surfaces contacting said current collectorplates.
 8. The polymer electrolyte fuel cell in accordance with claim 7,wherein portions of said mist discharge groove connecting with saidsupply and discharge manifolds for the fuel gas or oxidant gas areunevenly located to the lower side with respect to the direction ofgravity in which said cell stack is installed.
 9. The polymerelectrolyte fuel cell in accordance with claim 7, wherein said mistdischarge groove is subjected to hydrophilicity treatment.
 10. Thepolymer electrolyte fuel cell in accordance with claim 8, wherein awater absorbing material is provided in said mist discharge groove. 11.The polymer electrolyte fuel cell in accordance with claim 1, whereinsaid current collector plates have a mist discharge groove communicatingwith said supply and discharge manifolds for the fuel gas or oxidant gasin their surfaces contacting the conductive separator plates.
 12. Thepolymer electrolyte fuel cell in accordance with claim 11, whereinportions of said mist discharge groove connecting with said supply anddischarge manifolds for the fuel gas or oxidant gas are unevenly locatedto the lower side with respect to the direction of gravity in which saidcell stack is installed.
 13. The polymer electrolyte fuel cell inaccordance with claim 11, wherein said mist discharge groove issubjected to hydrophilicity treatment.
 14. The polymer electrolyte fuelcell in accordance with claim 11, wherein a water absorbing material isprovided in said mist discharge groove.
 15. A method of operation of thepolymer electrolyte fuel cell recited in claim 1, wherein thetemperature of the coolant introduced into said fuel cell is changeddepending of the amount of power generation of the fuel cell.
 16. Themethod of claim 15, wherein the temperature of the coolant is decreasedcontinuously or in stages as the amount of power generation of the fuelcell increases, and the temperature of the coolant is increasedcontinuously or in stages as the amount of power generation of the fuelcell decreases.
 17. A method of operation of the polymer electrolytefuel cell recited in claim 1, wherein the temperature of the coolantintroduced into the fuel cell is changed depending on the temperaturesof said separator plates, current collector plates or end plates.
 18. Amethod of operation of the polymer electrolyte fuel cell recited inclaim 1, wherein an exhaust path for the oxidant gas is closed andopened when output voltage of said cell becomes lower than apredetermined value in order to promote discharge of water content insaid exhaust path.